This invention relates to bulky cellulose fibers having a high liquid absorbency, an absorbent paper and an absorbent member each using said cellulose fibers, a topsheet using said cellulose fibers, and an absorbent article using said cellulose fibers.
Various absorbent articles, such as sanitary napkins, having an improved absorption of body fluids, have heretofore been proposed, and various improvements have also been made. Most of the studies for making the improvements have heretofore been directed to the improvement in the absorption rate, inhibition of reflow of the body fluid from the absorbent member to the surface, prevention of leakage, and reduction of stickiness to the user's body.
For example, as for the material for an absorbent member to be used in a sanitary napkin, it has been proposed to absorb and retain body fluids by using an absorbent polymer, which utilizes physicochemical effects, i.e. ionic osmotic pressures, in lieu of hydrophilic absorbent paper or pulp, which absorbs and retains the body fluids by the utilization of physically fine spaces. It has been reported that, with the proposed technique, the absorption capacity can be enhanced, and the reflow of the body fluids after being absorbed can be prevented from occurring. Actually, the proposed technique provides an improved absorbency of the sanitary napkin. Therefore, at present, absorbent members utilizing a combination of pulp and an absorbent polymer are used for most of sanitary napkins.
However, as is manifested by the fact that the main reason for dissatisfaction of the sanitary napkins resides in the leakage, even with the sanitary napkins utilizing the absorbent polymers, the requirement for the prevention of the leakage cannot be satisfied sufficiently.
Specifically, with the absorbent polymers which absorb and retain body fluids by utilizing the ionic osmotic pressures, a limitation is imposed on the body fluids absorption rate. Also, the absorbent polymers can absorb the body fluids only when they are wetted with the body fluids. Therefore, in cases where the highly absorbent polymers are used, it is necessary for pulp, or the like, having a high water absorption rate to be used in combination with the absorbent polymers. However, when the pulp is formed into a soft fluff absorbent layer as an absorbent member, the pulp absorbs blood spot-wise, and therefore the problems occur in that the diffusing capacity for the efficient utilization of the entire area of the absorbent member cannot be kept high.
Also, the pulp exhibits certain levels of compression and bending recoveries when it is in the dry state. However, when the pulp is in the wet state, the strength of the pulp decreases markedly, and the pulp exhibits little compression and bending recoveries. Therefore, when stress is applied to the wet pulp, compression deformation (hereinafter referred to as the "twist") occurs with the pulp, and the absorption spaces in the pulp reduce markedly. As a result, the body fluids, which have once been absorbed by the pulp, returns to the body side due to the twist, and the problems with regard to stickiness and leakage occur.
In addition, the reduction in the pulp fiber spaces due to the twist causes the resistance to transfer of the body fluids to the polymer to increase, and therefore the absorption efficiency of the polymer decreases. Moreover, synergistic effects occur from the reduction in the pulp fiber spaces due to reexcretion of the body fluids and from the reduction in the absorption spaces due to the swelling of the polymer. Due to such synergistic effects, the reabsorption rate of the entire absorbent member decreases markedly after the twist has occurred, and the leakage problems are often caused to occur.
Therefore, in order that the diffusing capacity of the pulp may be enhanced and the reduction in the absorption spaces due to the twist can be kept small, techniques for improving the diffusing capacity and the inhibition of reflow of the body fluids from the absorbent member to the surface by compressing the pulp and increasing the density of the pulp have been proposed in, for example, Japanese Utility Model Laid-open Application 49-143589, U.S. Pat. No. 3,017,304, and Japanese Patent Publication 54-36793 and 1-33176. However, none of these proposed techniques can solve the essential problems in that the strength of the pulp decreases markedly when the pulp is wetted. Conversely, these proposed techniques have the problems in that the resistance to transfer of blood to the polymer becomes very high due to the markedly reduced distance between pulp fibers, and as a result the absorption efficiency of the polymer used in combination with the pulp becomes low.
A sanitary napkin, which is very thin and has a high comfortableness and a high absorbency, is disclosed in, for example, Japanese Patent Laid-open Application 4-89053. The disclosed sanitary napkin aims at improving the absorption efficiency of a polymer and meeting both of the requirements for a small thickness and a high absorbency by combining a highly diffusing absorbent sheet and a specific polymer sheet with each other.
With the disclosed technique, a sanitary napkin, which has an improved absorption efficiency of the polymer, an absorbency is enhanced to some extent, and a small thickness can certainly be obtained. However, there are user's expectation and a need for sanitary napkins having a higher comfortableness and a higher absorbency. In particular, at present, there is a need for a very thin sanitary napkin, which has an excellent absorption performance and is free of the problems with regard to reflow of body fluids from an absorbent member to the surface, stickiness to the body, and leakage even during a long period of use under conditions of large excretion amounts.
Heretofore, in cases where the amount of a absorbent polymer used in an absorbent member is increased so that the absorption performance may be enhanced, the absorption performance can be enhanced as the amount of the absorbent polymer used in the absorbent member becomes large to a certain extent. However, if the amount of the absorbent polymer used in the absorbent member becomes very large, the spaces among the polymer particles after absorbing body fluids will become markedly small. As a result, the reabsorption rate becomes low, and therefore the absorption performance becomes bad.
Specifically, due to a gel blocking phenomenon of a polymer, a limitation has heretofore been imposed on the amount of the polymer used. Therefore, at present, an absorbent article cannot be obtained which perfectly satisfies the requirements for the absorption performance, such as inhibition of reflow of body fluids from the absorbent member to the surface, and inhibition of leakage, during the use under the conditions of a long period of use and a large excretion amount.
For the purpose of preventing the twist of wet pulp, various techniques have heretofore been proposed wherein the pulp cellulose is crosslinked by using an appropriate crosslinking agent, a decrease in the modulus of elasticity in the wet state is thereby restricted, and the twist and permanent set in fatigue are thereby reduced. For example, a crosslinked pulp having a bulky structure (i.e., a torsion structure) is proposed in Japanese Patent Laid-open Application 63-264971. Certainly, the absorbent member utilizing such a crosslinked pulp maintains the bulky structure not only in the dry state but also after absorbing body fluids and exhibits good spot absorbency and a high absorption rate.
However, it has been found that, with the absorbent member utilizing the crosslinked pulp, if it is used in an erroneous manner, the capacity of the entire absorbent member for retaining body fluids will decrease, reflow of the body fluids from the absorbent member to the surface will increase, and the body fluids will become apt to leak from the absorbent member. Specifically, in cases where the pulp is crosslinked, though its modulus of elasticity in the wet state can be improved, the capacity of the fibers themselves for absorbing body fluids is restricted due to the crosslinking. As a result, the capacity of the pulp itself for retaining the body fluids becomes low. Therefore, unless the capacity of the polymer for absorbing and retaining the body fluids is increased, the capacity of the entire absorbent member, which utilizes the crosslinked pulp, for absorbing and retaining the body fluids becomes low, reflow of the body fluids from the absorbent member to the surface increases, and the leakage inhibiting performance of absorbent member becomes bad.
For the purpose of preventing the gel blocking phenomenon of a polymer may be prevented and the absorption efficiency of the polymer may be enhanced, a technique for utilizing an absorbent member, which comprises a mixture of hydrophilic fibers, such as pulp, and a polymer, is proposed in, for example, Japanese Patent Laid-open Application 59(1984)-204956, and techniques for using a homogeneous layer, which is composed of a polymer and pulp, only at the bottom surface of an absorbent member are proposed in, for example, Japanese Patent Laid-open Applications 59-135149 and 63-109859. Also, for the purpose of preventing the gel blocking phenomenon of a polymer after absorbing body fluids more efficiently, techniques for constituting an absorbent member, in which the polymer concentration is reduced from the bottom surface towards the top surface of the absorbent member, i.e. a polymer concentration gradient is provided, are proposed in, for example, Japanese Patent Laid-open Applications 62-32950 and 62-45703.
The above-enumerated techniques relate to the constitution techniques for pulp and a polymer and have certain levels of effects of restricting the gel blocking phenomenon of a polymer and enhancing the absorption efficiency of the polymer. However, none of these proposed techniques can solve the essential problems in that, when the pulp is wetted, it becomes twisted, the fiber spaces become small, and the reabsorption rate becomes low.
U.S. Pat. No. 5,061,259 indicates that fine powder of a polymer constitutes a cause for the gel blocking phenomenon and discloses an absorbent member comprising a mixture of a polymer, the particle diameter distribution of which is defined so as to have a central particle diameter falling within the range of 400 .mu.m to 700 .mu.m, and hydrophilic fibers.
The aforesaid U.S. Patent specification exemplifies that a crosslinked pulp may be used as the hydrophilic fibers. However, the aforesaid U.S. Patent specification does not indicate anything about the effects obtained from the use of the crosslinked pulp, nor does it describe anything about the limitation of the crosslinked pulp as the hydrophilic fibers. Specifically, the technique disclosed in the aforesaid U.S. Patent specification cannot solve the essential problems in that, when the hydrophilic fibers are wetted, the spaces among the fibers become small, and the reabsorption rate becomes low.
Also, by the removal of fine powder of the polymer, the gel blocking phenomenon can be restricted to some extent. However, the gel blocking phenomenon of the polymer has the nature such that it cannot be eliminated only with the adjustment of the particle diameter distribution, and therefore the aforesaid problems of the polymer cannot be eliminated by the technique disclosed in the aforesaid U.S. Patent specification.
As for a topsheet for an absorbent article, the topsheet is required to have the liquid absorbing and permeating functions such that body fluids such as blood or urine can be quickly transferred to and absorbed by the absorbent member.
The transfer of the liquid from the topsheet to the absorbent member is achieved only when the topsheet and the absorbent member are in close contact with each other. If the topsheet and the absorbent member are located at a spacing from each other, the transfer of the liquid from the topsheet to the absorbent member will reduce markedly, and the amount of the liquid remaining on the topsheet will become large. As a result, a sticky feeling will be given to the user. Also, liquid diffusion and liquid flow will occur, and liquid leakage will be thereby caused to occur.
The problems with regard to the insufficient absorption due to separation of the topsheet and the absorbent member from each other occur particularly markedly in cases where the topsheet comprises a fiber aggregate, such as a nonwoven fabric, which has the absorbency by itself.
For example, in cases where the topsheet comprises a nonwoven fabric, if separation of the topsheet and the absorbent member from each other occurs, the liquid will remain stagnant in the topsheet and cannot be transferred to the absorbent member. Therefore, a sticky feeling will continue to be given to the user, and the absorbent article cannot be used comfortably.
In cases where the topsheet comprises a film type of material, such as a porous film or a porous net, which does not have the absorbency by itself, if the topsheet and the absorbent member are in close contact with each other, the liquid will not be apt to remain in the topsheet, a sticky feeling will not be given to the user, and the absorbent article can be used comfortably. However, if the topsheet and the absorbent member separate from each other, because the topsheet by itself has no absorbency, the liquid will not be absorbed by the topsheet and will flow along the surface of the topsheet. As a result, the liquid will leak from the absorbent article. This is a very serious drawback for the absorbent article.